This invention relates to imide-containing copolymers having improved rigidity, and more particularly to a method for improving the rigidity, water absorption characteristics and environmental resistance of imide-containing polymers. Still more particularly, the method of this invention provides imide-containing copolymers having increased rigidity and good retention of mechanical properties at elevated temperatures after exposure to humid environments.
Polyimides and amide-imide polymers are condensation polymers finding use in a wide range of applications such as adhesives, molding compositions, fibers, films, composites, laminates, etc., owing to a desirable combination of properties. Torlon.RTM. polyamide-imides, available from Amoco Performance Products, Inc., are examples of commercial polyamide-imides, while polyimides are available from a variety of commercial sources.
Despite the many desirable properties of such polymers, their utility in certain applications has been limited by moisture sensitivity, leading to loss of mechanical properties on exposure to high temperatures. In addition, many such polymers, when used as molding resins, do not attain a high level of mechanical properties without additional thermal tempering or similar post treatment of the molded article. So-called annealing or postcuring treatments of polyimide and of polyamide-imide or polyamide-amic acid fabricated parts such as are disclosed in commonly assigned U.S. Pat. No. 4,167,620 allow water liberated due to imidization and chain extension reactions as well as absorbed moisture to diffuse out of fabricated parts and may improve retention of mechanical properties. However, since polyimides and polyamide-imide resins tend to absorb water when exposed to humid environments, these treatments are not sufficient to permit the use of currently available, commercial polyimides and polyamide-imides in certain demanding applications. It is also known to add certain metal oxides to polyimides and polyamide-imides to tie up absorbed moisture as well as water liberated during imidization and chain extension reactions to avoid cracking and sacrifices in physical properties. Again, however, this approach does not yield sufficient improvement of presently available materials or prevent subsequent moisture absorption and further reduction in mechanical properties.
Although aromatic polyimides typically do not absorb water to as great an extent as polyamide-imides, the retention of physical properties on exposure to humid environments remains a potential problem for these materials. Moreover, the utility of aromatic polyimides, particularly for use as molding resins, is limited because their high glass transition temperatures ("Tg") may make melt processing impractical or even impossible.
U.S. Pat. No. 4,017,459, assigned to the Upjohn Company, discloses amide-imide polymers and copolymers prepared from 2,2-bis(4-(p-aminophenoxy)phenyl) propane and trimellitic anhydride halide or from 2,2-bis(4-(p-isocyanatophenoxy)phenyl) propane and trimellitic acid or anhydride. According to the patent, such polyamide-imides are melt processable, such as by injection molding, and useful in manufacture of articles such as gears, ratchets, clutch linings, bearings, pistons and cams and electrical components. In contrast, the patentee teaches that polyamides prepared from the above-named diamine and isophthalic acid, and polyimides prepared from that diamine and pyromellitic acid dianhydride or benzophenone tetracarboxylic acid dianhydride are intractable in the sense of lacking sufficient solubility for solution processing, lacking in melt processability or lacking both.
U.S. Pat. No. 4,111,906 and 4,203,922, both assigned to TRW, Inc., disclose that although processability of polyimides can be improved by using the same in predominantly polyamide-amic acid form and imidizing during a final fabrication step, such an approach is disadvantageous because voids in the final products result from water liberated due to the imidization reaction. These patents also state that chemical and thermal stability are improved by preparing polyimides from 2,2-bis(4-(p-aminophenoxy)phenyl) hexafluoropropane. According to the '906 patent, polyimides prepared from this diamine and a dianhydride are useful as coatings, adhesives and as a matrix for laminated glass or graphite structures. Polyimide foams prepared from pyromellitic acid dianhydride or other aromatic tetracarboxylic acid dianhydrides and such diamine in combination with a second aromatic diamine are disclosed in U.S. Pat. No. 4,535,101, assigned to Imi-Tech Corporation. Preparation of polyamides from the above-named diamine and diacids also is disclosed in the '906 patent. The abstracts of both the '906 and '922 patents mention polyamide-imides; however, no additional information is provided.
U.S. Pat. No. 4,340,697, assigned to Toray Industries, Inc., discloses melt processing difficulties with polyamide-imides and purports to remedy the same by blending with polyphenylene sulfide, polyamide, aromatic polyester, polyphenylene ether or a phenoxy resin.
According to this patent, polyamide-imides may contain, in addition to a repeating, main structural amide-imide unit, up to 50 mole percent amide or imide units, the latter being introduced into the polymer by replacing a portion of the aromatic tricarboxylic acid component with pyromellitic acid dianhydride or benzophenone tetracarboxylic acid dianhydride.
U.S. Pat. No. 4,599,383, assigned to NTN-Rulon Industries Co., Ltd. discloses compositions having improved water absorption properties containing a polyamide-imide resin in combination with a polyetherimide and a fluoro resin component.
U.S. Pat. No. 4,755,585, assigned to M & T Chemicals, Inc., discloses polyimides, polyamide acids, polyamide-imides, polyesterimides and polyesteramides containing at least 10 mole percent of a reaction product of an aromatic or aliphatic mono- or dianhydride and certain aromatic diamines having an unsubstituted or halogen- or hydrocarbyl-substituted, p-phenylene radical linked by like or different alkylene, alkenylene, sulfide or oxy groups to two unsubstituted or halogen- or hydrocarbyl-substituted, monovalent, aminophenyl radicals provided that the linking groups are not contemporaneously both sulfide or oxy. Such products are said to exhibit improved processing characteristics and thermal stability and to have utility in widespread applications. Two polyamide-imides and films thereof are demonstrated in the examples. Numerous anhydrides, dianhydrides and diamines are named in this publication and mixtures of anhydrides and dianhydrides are mentioned. It is also reported that mixtures of the above-described diamines with other diamines may be used. Interestingly, such other diamines are said to include 2,2-bis(4-(p-aminophenoxy)phenyl) propane and the corresponding sulfone, although the publication also mentions, with supporting citations, that polyimides prepared from such diamines and dianhydrides are insoluble and that polyamide-imides prepared from such diamines are of uncertain solubility and processability.
As shown by the patents discussed above, a great many polyamide-imides and polyimides are known. Although some resins, particularly polyimides, are known to have a high level of rigidity as reflected by the resin modulus, such resins tend to be quite intractable. More readily processable and tractable prior art polyimide and amide-imide polymers tend to have a greater ductility and flexibility and generally exhibit reduced rigidity, limiting their desirability for use in many more demanding applications requiring rigid, high modulus matrix resins that retain these characteristics after exposure to humid environments, and particularly at elevated temperatures, such as in structural composites.
A method for providing highly-rigid imide-containing resins that are readily processable and exhibit good moldability would be a useful advance in the art. Such resins, and particularly those which retain mechanical properties at elevated temperatures after exposure to humid environments, would find wide application in the form of composite materials and filled molding compounds as well as in neat resin form. Such resins would be useful in a variety of applications including, for example, electrically insulating molded goods and wire coatings, sporting goods and under-the-hood automotive components as well as for producing structural components for sports equipment, automobiles, buildings and aircraft.